ago
1.一般说来，用了ago不可用完成时态，如可以说She saw him a week ago，不可说She has seen him a week ago。但用了助动词或用在分词、动名词或不定式的形式里便可以用完成时态。例如：
- She must have seen him a week ago.
- She may have seen him a week ago.
- She cannot have seen him a week ago.
- Having seen him a week ago, she is always thinking of him.
- She speaks of having seen him a week ago.
- She seems to have seen him a week ago.
即使不用助动词，也不用在分词、动名词或不定式的形式里，偶尔也有用完成时态而用ago的。例如：I have long ago given up that habit. 这里语气重在“早已…”。
2.说现在的多少时间以前，该用ago，如He started five minutes ago 和He returned home twenty years ago。说过去某时的多少时间以前，该用before，如He told me he had seen her a few minutes before。不过英美也有人用ago的，往往用ago语气比较生动。说将来某时的多少时间以前，也该用before。例如：When you call at the shop again this afternoon, you may be told that the book has been sold only a few minutes before. 说任何某时（不是现在、过去或将来的某时）的多少时间以前，也该用before。例如：He often forgets what he has heard only a few minutes before. （这三种场合里的before改作previously或earlier也可以，但不及before普通。）
3.It is five years ago that he returned home和It was five years ago that he returned home都可以说。用is着重“到现在已有5年”的意思，假使在ago后面加上now，这意思更明显。不可说It was five years ago now that he returned home。
4.as long ago as 1864等表达方式里在第二个as后面没有in。可以说as long as forty years ago，不可说as long ago as forty years。
5.She died three years ago next Monday是“她在下一个星期一的3年以前死去”。假定下一个星期一是8月5日，那末她便是3年前的8月5日死的。It happened a fortnight ago tomorrow是“这件事发生在13天以前。”
6.“最近过去的第二个晚上”习惯说two evenings ago，说“最近过去的第三个夏天”习惯说three summers ago。
